Solvent extraction processes that use hydrocarbon solvents to extract bitumen from mined oil sands require little or no water, generate no wet tailings, and can achieve higher bitumen recovery than the existing Clark hot water extraction process or its variants. However, they requires a process for effective separation of solvent from the spent oil sands solids. This separated solvent may be recycled for use in extracting bitumen as the solvent extraction process continues. The separated oil sands solids may be used to form trafficable solids. Typical spent oil sands solids are in solid lump form, and contain water in the amount of about 5 weight percent, and solvent in the amount of about 5 to 10 weight percent. The solvent trapped in the spent oil sands solids is difficult to remove and recover.
FIG. 1 shows a flow diagram of a process for recovering solvent from spent oil sands solids, as described in Canadian Patent No. 2,794,373 (Wu et al.). The process involves drying the solids using superheated steam to vaporize solvent and water. The vapor is compressed and condensed in the hot side of a first heat exchanger to produce condensates including condensed hot water, condensed solvent, and uncondensed vapor. The condensed hot water and condensed solvent are separated from the uncondensed vapor in a first separator. Hot water is flowed through the cold side of the first heat exchanger to produce near-saturated steam. The near saturated steam is superheated in a second heat exchanger to produce the superheated steam for drying the solids. Uncondensed vapor from the first separator can be further condensed in a third heat exchanger to produce warm water, recovered solvent, and uncondensed off gas. The uncondensed off gas can be separated in a second separator. Some of the warm water is combined with the hot water to produce the near-saturated steam for superheating. The off gas is oil scrubbed or combusted prior to release to the atmosphere.
The process described by Wu et al. can effectively recover solvents form spent oil sands solids. However, a substantial amount of energy is needed to produce the condensates by compressing the vapor by a compression ratio in the range of 1.3 to 2.5. Compression is needed to raise the dew point of the vapor above 100° C. so that there is a temperature difference between condensing vapor on the hot side and vaporizing water on the cold side. However, compression of the vapor substantially raises the temperature of the vapor due to the adiabatic effect. While some of the heat in the vapor is transferred to the steam which is used in drying the solids, excess steam production wastes energy. Furthermore, higher vapor temperature needs to be brought down to its dew point prior to condensation in the heat exchanger. While the heat transfer coefficient for vapor condensation is high, the heat transfer coefficient for gas/vapor cooling is quite low. This increases the heat exchanging area required of the first heat exchanger, and hence the capital cost of the heat exchanger.
Commercial application of the solvent recovery process described in Wu et al. would benefit from improvements, including improvements in energy efficiency. For large-scale oil sands operations involving throughput rates on the magnitude of 8000 tonnes per hour of mined oil sands, even incremental gains in energy efficiency can substantially impact absolute energy consumption and operating costs.